Not a matter of something you have done. It could simpley be the time of day you are testing the
ph.
The best times to test are midday and late afternoon as
ph will have had time to come up from the early morning.
Just before the lights go on, the
ph will be at it's lowest point. Without light, algaes produce
CO2 much like houseplants and during the day produce
O2. The
CO2 acts against the
ph and lowers it. Once the lights come on, the same algaes begin
O2 production again and help increase the
ph.
I would also ask if you have a decent enough water flow at the surface for gas exchange and if the is no sump, are you using a glass cover on the tank?
Cheers
Steve